Continued support is requested for grant years 09 to 13 for a major program of development of electron spin resonance (ESR) instrumentation and methodology at the National Biomedical ESR Center. The proposal is totally dedicated to pulse or time domain ESR. In past years two unique pulse instruments have been built: a high-speed digital receiver, designated DRP-512, and a receiver with very fast digital phase detection, designated DPD. Our design philosophy has been to focus on Addition: rapid signal acquisition and preprocessing prior to the host computer, with emphasis on the highest possible rate of information flow. The DRP-512 is working well and the DPD has just been completed. Four research topics are addressed here: Three involve the use of the existing receivers in new ways, each with a modest level of development. These three are: (1) Saturation Recovery (SR) electron nuclear double resonance (ENDOR). The effect of induced nuclear resonance on the SR signal is determined. SR ENDOR studies of the structures of copper proteins and of free radicals will be conducted. (2) Multifrequency SR. Capability for S-band as well as X-band SR will be achieved. This will permit measurements of distances in biological systems in the range of 10-100 angstrom units. Separation of Heisenberg exchange and dipole-dipole interactions also will be possible. S-band SR ENDOR experiments are proposed. Multifrequency SR studies of weak Heisenberg exchange will be conducted. (3) Time Domain Spin Label Oximetry. Transient changes in O2 concentration that are caused by a copper vapor laser will be observed using both receivers. Measurements of the permeability of lipid bilayers to molecular oxygen and pulse SR measurements in the context of oxy-radical chemistry and reperfusion injury will be made. A fourth major topic: engineering development of a modernized digital receiver is proposed. Very rapid improvement in host computers, bus concepts, A/D converters and other digital components create copportunities for improved pulse ESR apparatus. The capabilities of the DRP-512 and DPD will be combined in a single "Hardware Development Microcomputer." The proposed instrument will permit a number of new types of experiments. Applications in Time Domain Spin-Label oximetry are particularly emphasized. The National Biomedical ESR Center is the prime facility in the world for development of ESR Instrumentation. ESR is a widely used basic spectroscopic technique that results in more than 2000 scientific papers per year, with about half in the life sciences over a wide variety of subjects.